refactor: modularize Beam into new projects and interfaces

- Introduced modularity by splitting Beam into new projects: Beam.Abstractions, Beam.Models, and Beam.Downloaders.
- Refactored existing classes into appropriate namespaces and projects.
- Replaced specific implementations with abstractions (e.g., SourceLinkBuilder to LinkBuilder, State to IState, etc.).
- Updated interfaces: added ITemplate, IArticleData, IDownloadReport, and others for improved extensibility.
- Removed deprecated classes like SourceLinkBuilder and StateChangerFactory.
- Enhanced link handling in downloaders by refactoring to use `string` over `SourceLink`.
- Consolidated shared logic under Beam.Abstractions.

Beam.Models/TableOfContentsData.cs

@@ -0,0 +1,14 @@
+using Beam.Models;
+
+namespace Beam.Models {
+    public record class TableOfContentsData : ArticleData {
+        /// <summary>
+        /// The link collection of the actual content
+        /// </summary>
+        public string[]? ContentLinks { get; set; }
+        /// <summary>
+        /// The link collection of all the Table Of Content pages for this specific resource.
+        /// </summary>
+        public string[]? PagesLinks { get; set; }
+    }
+}